Method for manufacturing field magnet

ABSTRACT

A field magnet manufacturing method where a bonded magnet&#39;s inner surface press-fitted in a yoke has a certain accuracy irrespective of the accuracy of the yoke&#39;s outer circumferential surface. A cylindrical bonded magnet from binding magnet particles with a thermosetting resin is fixed in a tubular yoke of magnetic material. The method includes reheating and softening the bonded magnet after thermal curing; and press-fitting in the bonded magnet after the softening step from a tapered portion on one end side of the yoke to press the bonded magnet&#39;s outer circumferential surface against the yoke&#39;s inner surface. The press-fitting includes feeding the bonded magnet relatively into the yoke while allowing a relative posture variation between the bonded magnet and the yoke so the bonded magnet&#39;s inner surface to be remolded into a shape along the inner surface of the yoke exhibits almost the same accuracy as the yoke&#39;s inner surface.

CROSS-REFERENCE

This application is a Divisional of application Ser. No. 17/595,349filed Nov. 15, 2021, which is a National Stage of InternationalApplication No. PCT/JP2020/021350 filed May 29, 2020, claiming prioritybased on Japanese Patent Application No. 2019-102775 filed May 31, 2019,the contents of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a fieldmagnet comprising a bonded magnet and a yoke (such as a case).

BACKGROUND ART

Permanent magnet field type motors (including generators/simply referredto as a “motor” or “motors”) are widely used as drive sources forvarious devices. To improve the performance, miniaturization, costreduction, etc. of such motors, it has been proposed to integrate acylindrical bonded magnet (permanent magnet for the field) in acylindrical yoke (e.g., the case of a motor). Descriptions relevant tothis are found in the following patent documents.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP2000-184642A-   Patent Document 2: WO2006/1304-   Patent Document 3: WO2011/126026-   Patent Document 4: JP2005-33844A-   Patent Document 5: WO2006/059603

SUMMARY OF INVENTION Technical Problem

These patent documents all propose a method of pressure-bonding a bondedmagnet to the inner surface of a yoke without using an adhesive.Specifically, Patent Document 1 discloses pressure-bonding a bondedmagnet and a yoke to each other using a phenomenon that the bondedmagnet disposed in the yoke oxidizes and expands by heating. PatentDocument 2 discloses press-fitting a molded body (bonded magnet beforethermal curing) directly into a yoke from a cavity and pressure-bondingthe two using springback of the molded body. Patent Document 3 disclosespress-fitting a molded body in a warm state directly into a case (yoke)and then pressure-bonding the two using an expansion amount of themolded body generated during thermal curing treatment (cure treatment).Patent Documents 1 to 3 are common in that a molded body is placed orfitted in a yoke (case) and then thermal curing treatment is performed.

Patent Documents 4 and 5 disclose reheating a molded body after thermalcuring treatment to a temperature equal to or lower than the glasstransition point and then press-fitting the molded body into a housing(yoke/case) to pressure-bond the two. The press-fitting performed inthis operation is not described in detail in these patent documents, butmay be performed while restraining the outer circumferential side of thehousing on the assumption that a high level of accuracy (such ascoaxiality, circularity, or cylindricity) is ensured between the outercircumferential surface and inner surface of the housing.

However, the accuracy of the outer circumferential surface itself of thehousing is basically unrelated to the performance of the motor. If ahigh level of accuracy is not required for the outer circumferentialsurface, the manufacturing cost of the housing (case/yoke) can bereduced, and the cost of the motor can also be reduced accordingly.

The present invention has been made in view of such circumstances, andan object of the present invention is to provide a novel manufacturingmethod capable of reducing the cost of a field magnet comprising abonded magnet and a yoke, unlike the conventional methods.

Solution to Problem

As a result of intensive studies to achieve the above object, thepresent inventors have succeeded in press-fitting a bonded magnet into ayoke (case) without restraining the outer circumferential side of theyoke. Developing this achievement, the present inventors haveaccomplished the present invention, which will be described below.

«Method for Manufacturing Field Magnet»

(1) The present invention provides a method for manufacturing a fieldmagnet configured such that a cylindrical bonded magnet is fixed in atubular yoke composed of a magnetic material. The bonded magnet isobtained by binding magnet particles with a thermosetting resin. Themethod comprises: a softening step for reheating and softening thebonded magnet after thermal curing treatment; and a press-fitting stepfor fitting in the bonded magnet after the softening step from one endside of the yoke to press an outer circumferential surface of the bondedmagnet against an inner surface of the yoke. The press-fitting stepincludes feeding the bonded magnet relatively into the yoke whileallowing a relative posture variation between the bonded magnet and theyoke.

(2) According to the manufacturing method of the present invention, theinner surface of the bonded magnet after the press fitting can beimparted with almost the same level of accuracy as the accuracy (such ascoaxiality or circularity) of the inner surface (inner cylinder portion)of the yoke regardless of the accuracy of the outer circumferentialsurface (outer cylinder portion) or the like of the yoke (such as acase). Therefore, if a yoke having a desired level of accuracy ensuredat least for the inner surface is prepared, a field magnet is obtainedin which a desired constant gap is ensured between the inner surface ofthe bonded magnet and the outer circumferential surface of a rotor(armature). As a result, the cost of the yoke (such as a case) can bereduced, and the cost of the field magnet and motor can also be reducedaccordingly.

The reason why such a field magnet can be obtained by the manufacturingmethod of the present invention is considered as follows. First, thebonded magnet before the thermal curing treatment (cure treatment)(simply referred to as a “molded body”) is configured such that amixture of magnet particles and a resin that has been softened or meltedby heating is molded in the cavity of a mold into a desired shape(cylindrical shape).

The molded body that has been taken out from the cavity and has not beencompletely cooled and solidified has insufficient strength and rigidityand has plasticity. In the molded body, therefore, slight deformation(strain) may occur due to handling such as when it is taken out from thecavity or when it is conveyed thereafter.

Bonded magnets are usually manufactured by being completely cooled(solidified) and then subjected to thermal curing treatment such asthrough a batch process without restoring the strain caused in such amolded body. The strain caused in the molded body therefore remains inthe bonded magnet subjected to the thermal curing treatment. The bondedmagnet after the thermal curing treatment has sufficiently high strengthor high rigidity, so if it is press-fitted into the yoke without anytreatment, the strain affects the accuracy of the inner surface of thebonded magnet in the yoke.

In the manufacturing method of the present invention, first, the bondedmagnet after the thermal curing treatment is reheated and softened. Thesoftened bonded magnet has the strength and rigidity required for pressfitting and also has plasticity. As this bonded magnet is fed along theinner surface of the yoke in a state in which the relative posturevariation with the yoke is allowed, the bonded magnet comes to a shapealong the inner surface of the yoke for which the accuracy is ensured.In other words, the bonded magnet is not strained by receiving extraexternal force or the like when fed by pressure, and the deformationbefore the reheating is restored. It is thus considered that the innersurface of the bonded magnet has almost the same level of accuracy asthat of the inner surface of the yoke.

«Others»

(1) In the present specification, for descriptive purposes, the bondedmagnet side is referred to as one end side while the yoke side isreferred to as the other end side. When the yoke is viewed, the sidefrom which the bonded magnet is introduced is the one end side while theopposite side is the other end side. When the bonded magnet is viewed,the tip side to be is fitted (fed) into the yoke is the other end sidewhile the rear end side (opposite side) is the one end side. When theyoke is placed above the bonded magnet and press-fitted, the lower sideis the one end side while the upper side is the other end side.

(2) Unless otherwise stated, a numerical range “x to y” as referred toin the present specification includes the lower limit x and the upperlimit y. Any numerical value included in various numerical values ornumerical ranges described in the present specification may be selectedor extracted as a new lower or upper limit, and any numerical range suchas “a to b” can thereby be newly provided using such a new lower orupper limit. As referred to in the present specification, “x to y μm”means x μm to y μm. The same applies to other unit systems.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view schematically illustrating a case, aring magnet, and assembling jigs for the case and the ring magnet.

FIG. 1B is a plan view of the other end side of the case.

FIG. 2A is a cross-sectional view schematically illustrating the initialstage of assembling the case and the ring magnet.

FIG. 2B is a cross-sectional view schematically illustrating the middlestage of assembling the case and the ring magnet.

FIG. 2C is a cross-sectional view schematically illustrating the latestage of assembling the case and the ring magnet.

FIG. 3 is a cross-sectional view illustrating a modified example of thecase.

FIG. 4 is a cross-sectional view schematically illustrating anotherexample of assembling the ring magnet to the case.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

One or more features freely selected from the matters described in thepresent specification can be added to the above-described features ofthe present invention. Which embodiment is the best or not is differentin accordance with objectives, required performance, and other factors.Methodological features regarding a manufacturing method can also befeatures regarding a product.

«Softening Step»

The softening step includes reheating and softening the bonded magnetsubjected to the thermal curing treatment. The softening may beperformed to such an extent that the bonded magnet has mechanicalproperties (such as rigidity and strength) that allow the bonded magnetto be press-fitted into the yoke and plasticity that allows the bondedmagnet to be in a cylindrical shape along the inner surface of the yoke.

The heating temperature of the bonded magnet is appropriately adjustedin accordance with the type of thermosetting resin, its mass ratio,thermal history, and the like. In general, the bonded magnet may beheated at a temperature lower than a thermal curing treatmenttemperature (To)+100 degrees. For example, that temperature may beT₀+70° C. or lower in an embodiment, T₀+40° C. or lower in anotherembodiment, T₀+10° C. or lower in still another embodiment, To or lowerin yet another embodiment, or T₀−20° C. or lower in still yet anotherembodiment. From another aspect, the temperature may be 40° C. or higherin an embodiment or 50° C. or higher in another embodiment. As anexample, when the thermosetting resin is a phenol novolac type epoxyresin, that temperature may be, for example, 270° C. to 40° C. in anembodiment, 240° C. to 40° C. in another embodiment, 210° C. to 50° C.in still another embodiment, 180° C. to 60° C. in yet anotherembodiment, 150° C. to 90° C. in still yet another embodiment, 140° C.to 100° C. in a further embodiment, or 130° C. to 110° C. in a stillfurther embodiment. As another example, when the thermosetting resin isa bisphenol A type epoxy resin, that temperature may be, for example,150° C. to 40° C. in an embodiment, 120° C. to 40° C. in anotherembodiment, 90° C. to 40° C. in still another embodiment, 80° C. to 50°C. in yet another embodiment, or 70° C. to 55° C. in still yet anotherembodiment.

If the reheating temperature is unduly high, the rigidity or the like ofthe bonded magnet may deteriorate, making the press fitting difficult.If the temperature is unduly low, the plasticity of the bonded magnetmay deteriorate, causing cracks in the bonded magnet or making itdifficult to follow the shape along the inner surface of the yoke. Theheating temperature of the bonded magnet as referred to in the presentspecification is the atmospheric temperature of a heating furnace or thelike. When the thermal curing treatment temperature varies, or when thethermal curing treatment is performed in multiple stages, the maximumtemperature during the thermal curing treatment may be set as areference (To) for the reheating temperature.

The heating time of the bonded magnet can be appropriately adjusted inaccordance with its size (thickness), the type and mass ratio of athermosetting resin, etc. For example, the bonded magnet may be reheatedfor 10 seconds to 1 hour in an embodiment or 20 seconds to 30 minutes inanother embodiment.

«Press-Fitting Step»

The press-fitting step includes feeding the bonded magnet relativelyinto the yoke while allowing the relative posture variation. When atleast one of the bonded magnet and the yoke is in an unrestrained state(non-fixed state), the relative posture variation is possible betweenthe two. At least a part of the bonded magnet and/or the yoke may besupported by a guide within a range in which the posture variation ispossible. In other words, the press-fitting step may be performed, forexample, in a completely unrestrained state (free state) of the outercircumferential side of the yoke or may otherwise be performed in astate in which a part of the outer circumferential side and/or the endportion side of the yoke is partially restrained by a guide or the like(e.g., a state in which, with reference to FIG. 1A to be describedlater, a flat surface portion 131 of a case 1 is preliminarily incontact with an inner bottom surface 33 a of a receiving jig 3).Likewise, the press-fitting step may be performed, for example, in acompletely unrestrained state (free state) of the inner side (e.g., aninner surface 22 a illustrated in FIG. 1A) and/or the outercircumferential side (e.g., an outer circumferential surface 22 billustrated in FIG. 1A) of the bonded magnet.

The posture variation may involve only translational movement, onlyrotational movement, or a combination of the two. The rotationalmovement may involve rotation around a certain starting point (such as acontact point with a jig) or rotation around another momentary center.The posture variation may involve, for example, lateral movement withrespect to the feeding direction (approximately the central axisdirection), so-called swinging, or the like. In essence, it is preferredthat a state be obtained in which extra force does not act on the bondedmagnet at the time of press fitting and the bonded magnet is smoothlyfitted in along the inner surface of the yoke. The posture variation maybe achieved by the movement of one of the yoke and the bonded magnet ormay also be achieved by the movement of the two (cooperation).

The inner surface side of the yoke, with which the tip end portion(other end portion) of the bonded magnet comes into contact at the timeof press fitting, is preferably formed with a tapered introductionportion (simply referred to as a “tapered portion”) whose diameterincreases toward the one end side. This enables smooth feeding of thebonded magnet into the yoke. The inclination angle of the taperedportion may be, for example, 5° to 12° in an embodiment or 6° to 10° inanother embodiment with respect to the central axis.

As substitute for the tapered portion or in addition to the taperedportion, another tapered portion such as a chamfer may be furtherprovided on the outer circumferential edge of the tip portion of thebonded magnet. Additionally or alternatively, even when the case has notapered portion, a tapered portion may be separately provided on the oneend side.

The interference, or tightening margin, is appropriately adjusted inaccordance with the thickness of the bonded magnet and yoke, therigidity of the bonded magnet, the application of the motor, etc. Theinterference may be such that at least the bonded magnet does not falloff from the yoke after the press-fitting step.

«Yoke»

The yoke is composed of a magnetic material. Provided that the innersurface of the yoke is a cylindrical surface, the outer circumferentialsurface may be or may not be a cylindrical surface. It suffices that atleast the inner surface of the yoke has a desired level of accuracy(e.g., circularity).

The yoke may have a flat surface portion orthogonal to the central axisof the inner surface. The flat surface portion serves, for example, as areference for the assembling accuracy (e.g., coaxiality) of the bondedmagnet, an armature, or the like. The flat surface portion may belocated, for example, on the other end side of the yoke (opposite sideto the introduction portion). When the case of a motor also serves asthe yoke, the cost and size of the motor can be reduced.

«Bonded Magnet»

The bonded magnet is obtained by subjecting a cylindrical molded bodycomprising magnet particles and a thermosetting resin to thermal curingtreatment.

(1) At least a part of the magnet particles is preferably rare earthmagnet particles. This allows the bonded magnet and accordingly a motorto have improved performance, reduced size, etc. The magnet particlesmay be isotropic or anisotropic. The bonded magnet composed of at leastisotropic magnet particles may be magnetized after being press-fittedinto the yoke. The bonded magnet composed of anisotropic magnetparticles may be formed of a molded body molded in magnetic fieldorientation. In this case, magnetization may be further performed.

The rare earth magnet particles are, for example, Nd—Fe—B-based magnetparticles, Sm—Fe—N-based magnet particles, Sm—Co-based magnet particles,or the like. The magnet particles are not limited to one type and may bea mixture of a plurality of types. For example, the plurality of typesof magnet particles may have different component compositions, differentparticle size distributions, or both.

(2) The thermosetting resin which is a binder resin is, for example, anepoxy resin, a phenol resin, an unsaturated polyester resin, an aminoresin, a polyamide resin, a polyimide resin, a polyamideimide resin, aurea-formaldehyde resin, a melamine resin, a urea resin, a diallylphthalate resin, polyurethane, or the like. In the presentspecification, when a curing agent, a curing aid, etc. are required forthe thermal curing, inclusion of them is referred to as a thermosettingresin.

The bonded magnet may appropriately contain a small amount of variousadditives that improve the wettability, adhesion, and the like betweenthe softened or melted thermosetting resin and the magnet powder.Examples of such additives include, for example, alcohol-basedlubricants and titanate-based or silane-based coupling agents.

(3) The conditions of the thermal curing treatment are appropriatelyadjusted in accordance with the type of the thermosetting resin, thesize of the bonded magnet, etc. When the thermosetting resin is an epoxyresin, the conditions may be adjusted within the ranges of heatingtemperature: 80° C. to 200° C. and heating time: 10 to 60 minutes,depending on the type of resin. The heating atmosphere may be an airatmosphere, but a non-oxidizing atmosphere (such as Ar, N₂, or vacuum)can suppress the deterioration of the bonded magnet (magnet particles)due to oxidation.

«Field Magnet»

The field magnet obtained by the manufacturing method of the presentinvention may be used for a direct current (DC) motor or an alternatingcurrent (AC) motor. The field magnet is typically used as a stator of asmall DC motor in which the armature is a rotor. Motors as referred toin the present specification also include generators.

EXAMPLES

The present invention will be described in more detail by exemplifying acase in which the ring magnet (bonded magnet) is attached to the case ofa motor (yoke).

«Configuration»

FIG. 1A illustrates a case 1 and a ring magnet 2 that constitute a fieldmagnet, and a receiving jig 3 and a feeding jig 4 that are used forassembling the case 1 and the ring magnet 2. For descriptive purposes,the up-down direction, the right-left direction, the feeding direction,and the lateral direction are respective directions illustrated in FIG.1A. In this case, the one end side as referred to in the presentspecification is on the lower side, and the other end side is on theupper side.

The case 1 is formed by molding a soft iron steel plate (magneticmaterial) into an approximately bottomed cylindrical shape. The case 1includes an opening portion 11, a cylindrical portion 12, and a bottomportion 13 in this order from the one end side. The opening portion 11has a tapered portion 114 (introduction portion) inside the other endside. The tapered portion 114 smoothly merges into the cylindricalportion 12 while reducing the diameter from the one end side (openingside) whose diameter is increased. The cylindrical portion 12 has aninner surface 12 a and an outer circumference surface 12 b. The innersurface 12 a has a desired level of accuracy (at least circularity), butthe accuracy of the outer circumferential surface 12 b is not ensured.

The bottom portion 13 has a shaft hole 132 at the center for supportingone end portion of the shaft of a rotor (armature). As illustrated inFIG. 1B, three flat surface portions 131 a to 131 c (collectivelyreferred to as a “flat surface portion 131”) are formed around the shafthole 132 on the other end outer surface of the bottom portion 13. Theflat surface portion 131 has a predetermined flatness and apredetermined squareness with respect to the central axis of thecylindrical inner surface 12 a and serves as an assembly reference ormeasurement reference for the field magnet or a motor.

The ring magnet 2 is composed of a bonded magnet obtained by subjectinga molded body to thermal curing treatment. The bonded magnet is formedby compression-molding a compound comprising rare earth magnet particlesand a thermosetting resin into a cylindrical shape. The ring magnet 2has a cylindrical portion 22, an annular end surface 21 on the one endside, and another annular end surface 23 on the other end side.

The cylindrical portion 22 has an inner surface 22 a and an outercircumferential surface 22 b, neither of which is not ensured to havecircularity or the like. However, the inner perimeter or thickness(lateral direction/radial direction) of the cylindrical portion 22 iswithin a predetermined range.

The receiving jig 3 is a bottomed cylindrical body composed of toolsteel. The receiving jig 3 includes an opening portion 31 on the one endside, a cylindrical portion 32, and a bottom portion 33. The innerdiameter of the cylindrical portion 32 is larger than the outer diameterof the cylindrical portion 12 of the case 1. Therefore, there is a gapbetween an inner surface 32 a of the receiving jig 3 and the outercircumferential surface 12 b of the case 1, and the case 1 is capable ofposture variation within the range of the gap.

The bottom portion 33 has an inner bottom surface 33 a, which is formedof a smooth flat surface having a predetermined flatness. The receivingjig 3 is arranged (fixed) so that its inner bottom surface 33 a isorthogonal to the vertical direction (feeding direction).

The feeding jig 4 is a stepped cylindrical body composed of tool steel.The feeding jig 4 includes a cylindrical base portion 41 and acylindrical interpolation portion 42 that extends upward (on the othersurface side) from the base portion 41. The outer diameter of the baseportion 41 is slightly smaller than the outer diameter of the ringmagnet 2 (outer circumferential surface 22 b). The outer diameter of theinterpolation portion 42 is smaller than the outer diameter of the baseportion 41 and further smaller than the inner diameter of the ringmagnet 2 (inner surface 22 a). The ring magnet 2 is placed on an annularlower surface 411 formed between the base portion 41 and theinterpolation portion 42 so that the end surface 21 is in contact withthe lower surface 411. There is also a gap between an outercircumferential surface 42 b of the interpolation portion 42 and aninner surface 22 a of the ring magnet 2, and the ring magnet 2 iscapable of posture variation within the range of the gap.

The length of the interpolation portion 42 in the up-down direction isshorter than that of the ring magnet 2, and an upper surface 421 of theinterpolation portion 42 does not protrude from the end surface 23 ofthe ring magnet 2 placed on the feeding jig 4. Therefore, the endsurface 23 of the ring magnet 2 can be fed to a position, or itsvicinity, at which the end surface 23 comes into contact with an innerbottom surface 13 a of the case 1. At that time, the upper surface 421of the interpolation portion 42 does not come into contact with theinner bottom surface 13 a of the case 1, and therefore the accuracy ofthe bottom portion 13 (flatness or the like of the flat surface portion131) is also maintained.

A hydraulic cylinder (not illustrated) is disposed on the one end sideof the feeding jig 4, and the feeding jig 4 moves up and down at adesired speed by controlling the pressure of oil supplied to thehydraulic cylinder.

At least one of the receiving jig 3 and the feeding jig 4 may also beallowed to change its posture with respect to the press-fittingdirection. For example, at least one of the jigs may be moved up anddown via a universal joint or the like extending in the press-fittingdirection. The posture variation of the receiving jig 3 and/or thefeeding jig 4 may be performed together with or as substitute for theposture variation between the case 1 and the ring magnet 2.

«Assembly»

(1) Softening Step

First, the ring magnet 2 is preliminarily heated in a heating furnace(air atmosphere) at a temperature lower than the thermal curingtreatment temperature. This allows the ring magnet 2 to exhibitplasticity while maintaining the strength and rigidity required for thepress fitting.

(2) Setting Step

Then, as illustrated in FIG. 1A, the softened ring magnet 2 is engagedwith the interpolation portion 42 from above and set on the feeding jig4. As described previously, there is a gap between the inner surface 22a of the ring magnet 2 and the interpolation portion 42 of the feedingjig 4, and the ring magnet 2 can therefore move laterally within therange of the gap.

Further, the opening portion 11 of the case 1 is engaged with the ringmagnet 2 from above. At this time, the case 1 is held in a state inwhich the inner surface of the tapered portion 114 is in contact withthe outer circumferential edge of the end surface 23 of the ring magnet2.

(3) Press-Fitting Step

In a state in which the ring magnet 2 is covered with the case 1, thehydraulic cylinder is operated to move the feeding jig 4 upward. Thisallows the bottom portion 13 and cylindrical portion 12 of the case 1 toenter the inside of the cylindrical portion 32 of the receiving jig 3.As described previously, there is a gap between the outercircumferential surface 12 b of the case 1 and the cylindrical portion32 of the receiving jig 3, and therefore the case 1 can also movelaterally within the range of the gap.

As the feeding jig 4 is further raised, as illustrated in FIG. 2A, theupper surface outer edge of the bottom portion 13 of the case 1 beginsto partially contact the inner bottom surface 33 a of the receiving jig3.

As the feeding jig 4 moves further upward, the upper surface outer edgeof the bottom portion 13 of the case 1 slides on the inner bottomsurface 33 a of the receiving jig 3 while the case 1 and the ring magnet2 undergo the posture variation or move in the lateral direction. Then,as illustrated in FIG. 2B, the flat surface portion 131 of the case 1comes into contact with the inner bottom surface 33 a of the receivingjig 3. Thus, the case 1 and the ring magnet 2 come into a state in whichthey are automatically aligned. That is, a state is obtained in whichthe central axes of the case 1 and ring magnet 2 are approximatelyaligned with each other along the direction (vertical direction)orthogonal to the flat surface portion 131 and the inner bottom surface33 a.

After the case 1 and the ring magnet 2 come into the aligned state, thering magnet 2 is fed into the case 1, as illustrated in FIG. 2C, as thefeeding jig 4 is further raised. Here, the ring magnet 2 has sufficientstrength and rigidity so as not to buckle and also has plasticity thatallows the ring magnet 2 to deform into a shape along the inner surface12 a of the case 1. The ring magnet 2 is therefore remolded along theinner surface 12 a of the case 1, in which the circularity is ensured,as the ring magnet 2 is fed into the case 1. As a result, the innersurface 22 a of the ring magnet 2 is imparted with almost the same levelof accuracy (circularity) as that of the inner surface 12 a of the case1.

The ring magnet 2 may be fed into the case 1 until the end surface 23comes into contact with the inner bottom surface 13 a or may otherwisebe stopped at a predetermined position before the contact.

Thus, a field magnet in which the ring magnet 2 is press-fitted in thecase 1 can be obtained. The inner surface 22 a of the field magnet has apredetermined level of accuracy (such as circularity) around the centralaxis orthogonal to the flat surface portion 131 of the case 1. As isclear from the above-described steps, the accuracy of the inner surface22 a has been ensured without depending on the accuracy or the like ofthe outer circumferential surface 12 b of the case 1.

MODIFIED EXAMPLES

(1) As substituted for the case 1, as illustrated in FIG. 3 , a case 5having an opening on the other end side can also be used. The previouslydescribed portions will be denoted by the same reference numerals asthose described above, and the description thereof will be omitted (hereand hereinafter).

When the case 5 is used, the ring magnet 2 is fed into the case 5 afteran annular end surface 53 (or its outer edge) of the case 5 slides onand comes into contact with the inner bottom surface 33 a of thereceiving jig 3. When the end surface 53 has the same level of accuracyas that of the flat surface portion 131, the accuracy (such ascoaxiality) of the inner surface 22 a of the ring magnet 2 is ensuredeven when the case 5 is used.

(2) As substitute for the case 1, as illustrated in FIG. 4 , a case 6having a bottom portion 63 can also be used. The bottom portion 63 hasthree recesses around a shaft hole 632 (corresponding to the shaft hole132), and flat surface portions 631 a to 631 c (collectively referred toas a “flat surface portion 631”) are formed on the upper surface side ofrespective recesses (the flat surface portion 631 c is omitted).Likewise the flat surface portion 131, the flat surface portion 631ensures a desired level of accuracy (squareness) with respect to thecentral axis.

The receiving jig 3 is changed to a jig having protrusions 71 a to 71 c(collectively referred to as a “protrusion 71”) that each come intocontact with the flat surface portion 631 (the protrusion 71 c isomitted). When the ring magnet 2 is fed into the case 6 using thefeeding jig 4 while the flat surface portion 631 of the case 6 issupported by the protrusion 71, the accuracy of the inner surface 22 aof the ring magnet 2 is ensured as in the previously described cases.

EVALUATION EXAMPLES (1) Case and Ring Magnet

Using the actually manufactured case 1 and ring magnet 2, the effects ofthe above-described softening step and press-fitting step wereconfirmed.

For the case 1, a bottomed cylindrical body made of cold-rolled steelplate (outer diameter: 04.0 mm, inner diameter: 00.0 mm, length: 70.0mm) was prepared. The tapered portion 114 (introduction portion)provided on the inner surface side was set to have an inclination angleof 8° with respect to the central axis. The circularity of the innersurface 12 a of the case 1 was set to 0.10 mm. The circularity wasmeasured in accordance with JIS B0021 (here and hereinafter).

The ring magnet 2 was manufactured as follows. A compound comprisingNdFeB-based rare earth anisotropic magnet particles (available fromAICHI STEEL CORPORATION, MAGFINE magnet powder MF15P) and an epoxy resinwas used as the raw material. The amount of epoxy resin with respect tothe entire compound was 3 mass %. The epoxy resin was a phenol novolactype, and its thermosetting temperature was 150° C.

The compound was compressed while being heated in the cavity of a mold(150° C.×130 MPa×6 seconds) to obtain a cylindrical molded body (outerdiameter: 00.2 mm×inner diameter: φ28.1 mm×length: 30 mm). This moldedbody was heated in a heating furnace of an air atmosphere and subjectedto thermal curing treatment (150° C.×40 minutes). The ring magnet 2composed of the bonded magnet was thus prepared.

(2) Assembly

First, the ring magnet 2 after the softening step was incorporated intothe case 1 using the receiving jig 3 and feeding jig 4 illustrated inFIG. 1A (press-fitting step). In the softening step, the ring magnet 2was heated in a heating furnace of an air atmosphere (150° C.×30seconds). The press-fitting step was performed with a load of 985 N anda moving speed of 30 mm/sec.

The inner surface 22 a of the ring magnet 2 thus incorporated in thecase 1 had a circularity of 0.12 mm.

(3) Comparative Example

When the above-described press-fitting step was performed on the ringmagnet 2 which was not subjected to the softening step, cracks sometimesoccurred on the inner surface 22 a of the ring magnet 2. When the ringmagnet 2 after the above-described softening step was fed into the case1 without allowing the posture variation between the case 1 and the ringmagnet 2, the load was abnormal and the press fitting was not able to beperformed.

(4) Evaluation

From each measurement result (circularity), it has been confirmed thatthe inner surface 22 a of the ring magnet 2 can have a desired accuracyby press-fitting the softened ring magnet 2 into the case 1 in a statein which the posture variation is allowed. On the contrary, it has beenfound that when the softening step is omitted or the press-fitting stepis performed without allowing the posture variation, it is difficult toproperly press-fit the ring magnet 2 into the case 1.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1 Case (York)    -   2 Ring magnet (bonded magnet)    -   3 Receiving jig    -   4 Feeding jig

1. A method for manufacturing a field magnet configured such that acylindrical bonded magnet is fixed in a tubular yoke composed of amagnetic material, the bonded magnet obtained by binding magnetparticles with a thermosetting resin, the method comprising: a softeningstep for reheating and softening the bonded magnet after thermal curingtreatment; and a press-fitting step for fitting in the bonded magnetafter the softening step from one end side of the yoke to press an outercircumferential surface of the bonded magnet against an inner surface ofthe yoke, the yoke being supported by a receiving jig during fitting,the press-fitting step including feeding the bonded magnet relativelyinto the yoke by a feeding jig while allowing a relative posturevariation between the bonded magnet and the yoke, wherein the yoke has aflat surface portion orthogonal to a central axis of the inner surfaceon another end side, and wherein the receiving jig includes protrusionsconfigured to support the flat surface portion of the yoke.
 2. Themethod for manufacturing a field magnet according to claim 1, whereinthe softening step includes heating the bonded magnet at a temperaturelower than a temperature that is higher than the thermal curingtreatment temperature (T₀) by 100° C. (T₀+100° C.).
 3. The method formanufacturing a field magnet according to claim 1, wherein thepress-fitting step is performed without restraining the outercircumferential side of the yoke.
 4. The method for manufacturing afield magnet according to claim 1, wherein the inner surface side of theyoke is formed with a tapered introduction portion whose diameterincreases toward the one end side.
 5. The method for manufacturing afield magnet according to claim 1, wherein the yoke is a case of amotor.